1. Field of the Invention
The present invention relates to an electromagnetic actuator, a method of manufacturing a part of an electromagnetic actuator, and a lithographic apparatus comprising an electromagnetic actuator.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Movable parts to be used in a lithographic apparatus, such as patterning device or a substrate (but not limited thereto), are held on a respective movable support during processing. The support is moved in one or more degrees of freedom using an appropriate actuator. Conventionally, such an actuator is of an electromagnetic type having a first part and a second part being movable relative to each other, the first part comprising at least one electric coil, and the second part comprising at least one permanent magnet interacting with the at least one coil. Either the first part or the second part may be coupled to the movable support. The actuator may be an ironless or an iron-core actuator, where the term “iron” represents any magnetizable material.
Conventionally, a cooling of an electric coil of an actuator has been accomplished by a thin plate-like cooling structure of a thermally conducting material in which cooling channels have been formed, such as a sandwich structure. A cooling medium, such as a gas or a liquid, is made to flow through the channels. The cooling structure is placed on, and connected to the coil, in particular at a side thereof facing the at least one permanent magnet, in order to remove heat generated in the coil to prevent the heat from reaching a permanent magnet. A heating of a permanent magnet may lead to a loss of magnetization, and may even lead to an irreversible loss of magnetization, which is to be avoided.
However, it is a trend to decrease the dimensions of actuators, and to increase the electrical load of the at least one coil such as to make the actuator produce higher forces. An increased electrical load leads to an increased generation of heat in the at least one coil. A cooling structure must have reduced dimensions like the actuator. Such a cooling structure placed on the coil does not suffice to remove the heat effectively to keep the at least one permanent magnet substantially free from a heat load. In fact, without further measures the at least one permanent magnet would be heated up through radiation and convection (via air present in an air gap of the actuator between the first part and the second part thereof), thereby changing or reducing the performance of the actuator, which is undesirable.